1. Field of the Invention:
This invention relates primarily to dynamoelectric machinery having a superconductive winding, and, more specifically, this invention relates to dynamoelectric machinery utilizing superconductive field or excitation windings with a ferromagnetic field structure.
2. Description of the Prior Art:
Interest in the use of cryogenic technology in dynamoelectric machinery has recently become quite intense. Since 1911 it has been recognized that when certain materials are cooled to near absolute zero they exhibit a complete loss of electrical resistance. Although attempts have been made from time to time to make practical application of this theoretical knowledge, the phenomenon of superconductivity has remained essentially a scientific curiosity.
It is only within the last decade or so that the practical utilization of the resistanceless character of superconductive materials at cryogenic temperatures has been studied with any real intensity as a viable possibility for use in the electrical machinery field. Several of the more technologically advanced nations of the world are now seriously investigating the possibility of utilizing the phenomenon of superconductivity in electrical motors and generators.
There are a number of reasons for the relatively recent growth of interest in superconductivity. Among these reasons is the necessity generated by technological advances that greater and greater power requirements be met by single units. Conventional design techniques seem to be asymptotically approaching a point where small additional power outputs can be reached only by excessive size increases, with the accompanying manufacturing and cooling problems. At the same time, the additional space requirements resulting from such increased power capabilities, when conventional designs can provide such capabilities, are becoming less and less acceptable. In fact, there is even considerable pressure for the reduction of the size of machines having power outputs that can be realized by conventional designs, both as a practical matter resulting from the sharply inflationary costs of materials and as a public service resulting from the sociological emphasis on ecological considerations involved in the procuring and processing of such materials. In response to these pressures and the development of intrinsically stable superconductors, much consideration is being given to the possibility of utilizing superconductive machinery to meet some of the power requirements of the future.
The great benefit of superconductive windings in dynamoelectric machinery is that, as a result of the large current that can be handled with virtually no resistance heating and subsequent power loss, extremely high flux densities can be produced in the machines. As a result of these very high flux densities, the loss or inefficient use of some magnetic flux is not as important as in conventional machines, where a limited supply of magnetic flux is involved. Accordingly, efforts directed to the use of the superconductive effect in rotating machinery have been concentrated on the removal of iron to decrease the size and weight of the machine. As a result, prior art work in this field has been devoted to production of machines utilizing essentially all air gap arrangements.
Although superconductive windings produce extremely high strength magnetic fields, use of an all air gap construction still presents considerable difficulties with respect to the obtaining of sufficient magnetic flux at the positions required for efficient motor or generator operation for all the modes of operation. The use of iron or other ferromagnetic material to aid in the distribution and control of the dynamoelectric machine magnetic fluxes has not been considered a viable approach, due to the fact that the ferromagnetic material would be saturated at the high field strengths resulting from the large currents in the superconductor.
With respect to direct current (DC) machines, the use of all air gap construction has resulted in less efficient operation than desired and has been accompanied by significant commutation problems. Therefore, even though the weight and size of DC machines may be considerably reduced by the use of superconductive windings in an all air gap construction, the ratio of efficiency to weight and volume has not evidenced as dramatic an increase as would appear to be theoretically possible by the use of superconductive windings.
With respect to alternating current (AC) machines, an all air gap arrangement utilized in connection with a superconductive excitation winding will perform satisfactorily at synchronous speeds. However, the relatively low frequency AC fields present during asynchronous operation, when the machine is starting or transient conditions are encountered, will penetrate into the superconductive winding and cause excessive eddy current and hysteresis losses. Thus, prior art devices have had to include a separate flux shield, such as that disclosed in U.S. Pat. No. 3,679,920 -- MacNab et al, issued July 25, 1972.